How can I with mysqli make a query with LIKE and get all results?

Here's how you properly fetch the result

$param = "%{$_POST['user']}%";
$stmt = $db->prepare("SELECT id,username FROM users WHERE username LIKE ?");
$stmt->bind_param("s", $param);
$stmt->execute();
$stmt->bind_result($id,$username);

while ($stmt->fetch()) {
  echo "Id: {$id}, Username: {$username}";
}

or you can also do:

$param = "%{$_POST['user']}%";
$stmt = $db->prepare("SELECT id, username FROM users WHERE username LIKE ?");
$stmt->bind_param("s", $param);
$stmt->execute();

$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    echo "Id: {$row['id']}, Username: {$row['username']}";
}

I hope you realise I got the answer directly from the manual here and here, which is where you should've gone first.

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Updated

From comments it is found that LIKE wildcard characters (_and %) are not escaped by default on Paramaterised queries and so can cause unexpected results.

Therefore when using "LIKE" statements, use this 'negative lookahead' Regex to ensure these characters are escaped :

$param = preg_replace('/(?<!\\\)([%_])/', '\\\$1',$param);

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As an alternative to the given answer above you can also use the MySQL CONCAT function thus:

$stmt = $db->prepare("SELECT id,Username FROM users WHERE Username LIKE CONCAT('%',?,'%') ");
$stmt->bind_param("s", $param);
$stmt->execute();

Which means you do not need to edit your $param value but does make for slightly longer queries.